1. Field of the Invention
The invention relates to a surface-emitting laser module, an optical scanner device and an image forming apparatus.
2. Description of the Related Art
There has been an increasing demand for multi-color image forming apparatuses capable of producing high-resolution images. The printing speeds of such multi-color image forming apparatuses have been increased every year so that the multi-color image forming apparatuses are utilized for simplified printing in on-demand printing systems. Specifically, such a multi-color image forming apparatus is, for example, provided with a two-dimensional laser array element including two-dimensionally arranged surface-emitting lasers such that sub-scanning intervals of recording density on photoreceptors are adjusted approximately to 1/n. Accordingly, the multi-color image forming apparatus is capable of forming a n*m dot matrix configuration as a pixel unit.
In optical systems including the surface-emitting laser elements or semiconductor laser elements, laser light may be fluctuated by feedback light, which is light reflected from lenses or a glass cover returning to the original laser elements. Examples of such laser-light fluctuation include various light fluctuation types such as high-speed laser-light fluctuation occurring in nsec order and laser-light fluctuation occurring in msec order. Since the surface-emitting lasers generally have a high mirror reflectance, they are considered to have high resistance to the feedback light. However, recent findings suggest the surface-emitting lasers are not necessarily highly resistant to the feedback light. In particular, in a case where the optical system includes a surface-emitting laser array including plural surface-emitting lasers, laser light emitted from a surface-emitting laser is reflected to an adjacent surface-emitting laser to fluctuate its laser light.
In order to suppress the effect of such feedback light, Japanese Patent Application Publication No. 2005-252032 (hereinafter referred to as “Patent Document 1”) discloses a technology to increase feedback light resistance of the surface-emitting laser element. In the technology disclosed in the patent. Document 1, relaxation oscillation frequency in the resonator is set to exceed an optical communication frequency which modulates a laser beam output from the surface emitting laser element by applying the modulation doping of carbon to a barrier layer of the active layer and increasing a differential gain. Further, Japanese Patent Application Publication No. 2005-86027 (hereinafter referred to as “Patent Document 2”) discloses a technology to enhance the feedback light resistance of the surface-emitting laser by providing a laser-light absorption layer to partially absorb laser light in the surface-emitting laser element.
Moreover, Japanese Patent No. 4351965 (hereinafter referred to as “Patent Document 3”) discloses a technology to lower an adverse effect of the feedback light. In the technology disclosed in Patent Document 3, optical fibers are slanted at 2 degrees or more to the surface-emitting laser to avoid the feedback light reflected from end surfaces of the optical fibers. The methods of slanting the end surfaces of the glass cover or optical fibers are generally used for reducing the adverse effect of the feedback laser light. However, the degrees at which the end surfaces of the optical fibers are simply slanted may be limited by its design configuration, and hence, it is preferable that the maxima effect may be obtained in reducing the feedback light with the minim a slant degrees.
However, with the technologies disclosed in Patent Documents 1 through 3, it appears to be difficult to reduce the light fluctuation to a predetermined amount or less and thus it may be difficult to produce a surface-emitting laser module capable of lowering the light fluctuation to a predetermined amount or less.